Which interaction is denoted as (+, -) in ecological terms?
Parasitism is represented by the sign pair (+, −) because the parasite gains resources or reproductive success while the host suffers reduced fitness. A tapeworm obtains nutrients from its vertebrate host, for example, while imposing costs through tissue damage, nutrient loss, immune activation, or reduced survival and fecundity. The signs describe average effects on each participant relative to its performance without the interaction; they do not imply that every encounter has an identical outcome. Predation and herbivory are also broadly positive–negative consumer interactions, but parasitis
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13